Process for reactivating a deactivated dehydrocyclodimerization catalyst

ABSTRACT

This invention relates to a process for reactivating a dehydrocyclodimerization catalyst. Dehydrocyclodimerization catalysts which contain an aluminum phosphate binder can be deactivated when they are exposed to hydrogen at temperatures above 500° C. The instant process restores substantially all of the catalyst&#39;s lost activity. The process involves treating the catalyst with an aqueous solution of a weakly acidic ammonium salt or a dilute acid solution at a temperature of about 50° to about 100° C. for a time of about 1 to about 48 hours. An ammonium nitrate solution is preferred. Next the catalyst is calcined at a temperature of about 500° to about 700° C. for a time of about 1 to about 15 hours to provide a reactivated catalyst. The catalyst can be reactivated several times using this process.

FIELD OF THE INVENTION

The present invention relates to a process for reactivating a spent dehydrocyclodimerization catalyst, thereby extending the useful life of the catalyst.

BACKGROUND OF THE INVENTION

Dehydrocyclodimerization is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce a high yield of aromatics and hydrogen, with a light ends byproduct and a C₂ -C₄ recycle product. This process is well known and is described in detail in U.S. Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500° C., using dual functional catalysts containing acidic and dehydrogenation components. The acidic function is usually provided by a zeolite which promotes the oligomerization and aromatization reactions, while a non-noble metal component promotes the dehydrogenation function. One specific example of a catalyst disclosed in U.S. Pat. No. 4,746,763 consists of a ZSM-5 type zeolite, gallium and a phosphorus containing alumina as a binder.

The conditions used for the dehydrocyclodimerization reaction result in catalyst deactivation which is believed to be caused by excessive carbon formation (coking) on the catalyst surface. After several days (from about 3 to 10 depending on the operating temperature) enough activity has been lost due to coke deposition necessitating regeneration of the catalyst. Regeneration involves burning or oxidizing the coke present on the catalyst at elevated temperatures. In addition to loss of activity due to coke formation, applicants have observed that catalysts containing a phosphorus modified alumina as a binder are gradually deactivated (over a period of time from several months to about a year) by exposure to hydrogen at temperatures greater than 500° C. Applicants have found that the loss of activity due to hydrogen exposure cannot be restored by regeneration means, i.e., burning or oxidation at elevated temperatures. Therefore, there is a need for reactivating catalysts that have lost activity due to exposure to hydrogen above 500° C.

Applicants have met this need by providing a reactivation process in which the catalyst is treated with aqueous solution of a weakly acidic ammonium salt or a dilute acid solution at a temperature and for a time sufficient to remove any deleterious species that have formed on the catalyst. This treatment removes some aluminum and phosphorus (and small amounts of silicon) from the catalyst as evidenced by analysis of the wash water. Next, the catalyst is oxidized in air, i.e., calcined, at a temperature and for a time sufficient to fully reactivate the catalyst.

As used in this application, regeneration refers to the process of restoring lost activity due to coke formation, while reactivation refers to the process of restoring lost activity due to hydrogen exposure.

SUMMARY OF THE INVENTION

As stated, the instant invention relates to a process for reactivating a dehydrocyclodimerization catalyst comprising a zeolite having a Si:Al ratio greater than about 10 and a pore diameter of about 5-6 Å, an aluminum phosphate binder and gallium, said catalyst having been deactivated by exposure to hydrogen at a temperature above 500° C., the process comprising treating the catalyst with an aqueous solution of a weakly acidic ammonium salt or a dilute acid solution at a temperature of about 50° to about 100° C. for a time of about 1 to about 48 hours and then calcining the treated catalyst at a temperature of about 500° to about 700° C. for a time of about 1 to about 15 hours, thereby providing a reactivated catalyst.

DETAILED DESCRIPTION OF THE INVENTION

As stated, this invention relates to a process for reactivating a deactivated dehydrocyclodimerization catalyst. The catalysts which require reactivation as described herein are those that contain a phosphorus modified alumina (hereinafter aluminum phosphate) as a binder in addition to a zeolite and a gallium component. As will be described in detail herein, these catalysts require reactivation because they have lost activity owing to exposure to hydrogen at temperatures above 500° C. These catalysts are well known in the art and their preparation is also well known as shown by U.S. Pat. No. 4,629,717 which is incorporated by reference.

The zeolites which may be used are any of those which have a Si:Al ratio greater than about 10 and preferably greater than 20 and a pore diameter of about 5 to 6 Angstroms. Specific examples of zeolites which can be used are the ZSM family of zeolites. Included among this ZSM family are ZSM-5, ZSM-8, ZSM-11, ZSM-12 and ZSM-35. The preparation of these ZSM-type zeolites is well known in the art and generally are prepared by crystallizing a mixture containing an alumina source, a silica source, an alkali metal source, water and a tetraalkyl ammonium compound or its precursor. The amount of zeolite present in the catalyst can vary considerably but usually is present in an amount from about 30 to about 90 weight percent and preferably from about 50 to about 70 weight percent of the catalyst.

A second component of the catalyst is an aluminum phosphate. The phosphorus may be incorporated with the alumina in any acceptable manner known in the art. One preferred method of preparing this aluminum phosphate is that described in U.S. Pat. No. 4,629,717. The technique described in the '717 patent involves the gellation of a hydrosol of alumina which contains a phosphorus compound using the well-known oil drop method. Generally this technique involves preparing a hydrosol by digesting aluminum in aqueous hydrochloric acid at reflux temperatures of about 80° to 105° C. The ratio of aluminum to chloride in the sol ranges from about 0.7:1 to about 1.5:1 weight ratio. A phosphorus compound is now added to the sol. Preferred phosphorus compounds are phosphoric acid, phosphorous acid and ammonium phosphate. The relative amount of phosphorus and aluminum expressed in molar ratios ranges from about 1:1 to 1:100 on an elemental basis.

The resulting aluminum phosphate hydrosol mixture is now gelled. One method of gelling this mixture involves combining a gelling agent with the mixture and then dispersing the resultant combined mixture into an oil bath or tower which has been heated to elevated temperatures such that gellation occurs with the formation of spheroidal particles. The gelling agents which may be used in this process are hexamethylenetetraamine, urea or mixtures thereof. The gelling agents release ammonia at the elevated temperatures which sets or converts the hydrosol spheres into hydrogel spheres. The spheres are then continuously withdrawn from the oil bath and typically subjected to specific aging and drying treatments in oil and in ammoniacal solution to further improve their physical characteristics. The resulting aged and gelled particles are then washed and dried at a relatively low temperature of about 93° C. to about 149° C. (200°-300° F.) and subjected to a calcination procedure at a temperature of about 450° C. to about 703° C. (850°-1300° F.) for a period of about 1 to about 20 hours. The amount of phosphorus containing alumina component present (as the oxide) in the catalyst can range from about 10 to about 70 weight percent and preferably from about 30 to about 50 weight percent.

The zeolite and aluminum phosphate binder are mixed and formed into particles by means well known in the art such as gellation, pilling, nodulizing, marumerizing, spray drying, extrusion or any combination of these techniques. A preferred method of preparing the zeolite/aluminum phosphate support involves adding the zeolite either to an alumina sol or a phosphorus compound, forming a mixture of the alumina sol/zeolite/phosphorus compound which is now formed into particles by employing the oil drop method described above. The particles are calcined as described above to give a support.

Another component of the catalysts which can be reactivated is a gallium component. The gallium component may be deposited onto the support in any suitable manner known to the art which results in a uniform dispersion of the gallium. Usually the gallium is deposited onto the support by impregnating the support with a salt of the gallium metal. The particles are impregnated with a gallium salt selected from the group consisting of gallium nitrate, gallium chloride, gallium bromide, gallium hydroxide, gallium acetate, etc. The amount of gallium which is deposited onto the support varies from about 0.1 to about 5 weight percent of the finished catalyst expressed as the metal.

The gallium compound may be impregnated onto the support particles by any technique well known in the art such as dipping the catalyst into a solution of the metal compound or spraying the solution onto the support. One preferred method of preparation involves the use of a steam jacketed rotary dryer. The support particles are immersed in the impregnating solution contained in the dryer and the support particles are tumbled therein by the rotating motion of the dryer. Evaporation of the solution in contact with the tumbling support is expedited by applying steam to the dryer jacket. After the particles are completely dry, they are heated under a hydrogen atmosphere at a temperature of about 500° to about 700° C. for a time of about 1 to about 15 hours. Although a pure hydrogen atmosphere is preferred to reduce and disperse the gallium, the hydrogen may be diluted with nitrogen. Alternatively, it is envisioned that the reduction and dispersion can be done in situ in the actual reactor vessel used for dehydrocyclodimerization by using either pure hydrogen or a mixture of hydrogen and hydrocarbons. Next the hydrogen treated particles are heated in air and steam at a temperature of about 400° to about 700° C. for a time of about 1 to about 10 hours. The amount of steam present in the air varies from about 1 to about 40 percent. This provides a catalyst with well dispersed gallium.

Although the catalysts can be used as is, it is preferred that they be treated with an aqueous solution of a weakly acidic ammonium salt or a dilute acid solution. The purpose of this treatment, as will be discussed in detail below, is to minimize the deactivation caused by exposure to hydrogen. The ammonium salts which can be used include ammonium chloride, ammonium acetate, ammonium nitrate and mixtures thereof. The total concentration of these salts can vary from about 0.1 to about 5 molar. The acids which can be used include hydrochloric, acetic, nitric and sulfuric acid. Although concentrated acids could be used, they would degrade the zeolite and the integrity of the particles as well as removing the undesirable aluminum phosphorus species. Thus, it is desirable to use dilute acids which have a molarity from about 0.1 to about 5 moles/liter. Of these treatment solutions, it is preferred to use an ammonium nitrate solution.

The treating solution is contacted with the calcined catalyst particles at a temperature of about 50° to about 100° C. for a time of about 1 to about 48 hours. After this treatment, the particles are separated from the aqueous solution, dried and calcined at a temperature of about 500° to about 700° C. for a time of about 1 to about 15 hours, thereby providing the catalyst of the instant invention.

The dehydrocyclodimerization conditions which are employed vary depending on such factors as feedstock composition and desired conversion. A desired range of conditions for the dehydrocyclodimerization of C₂ -C₆ aliphatic hydrocarbons to aromatics include a temperature from about 350° C. to about 650° C., a pressure from about 1 to about 20 atmospheres, and a liquid hourly space velocity from about 0.2 to about 5 hr⁻¹. The preferred process conditions are a temperature in the range from about 400° to about 600° C., a pressure in or about the range from 2 to 10 atmospheres and a liquid hourly space velocity of between 0.5 to 3.0 hr⁻¹. It is understood that, as the average carbon number of the feed increases, a temperature in the lower end of the temperature range is required for optimum performance and conversely, as the average carbon number of the feed decreases, the higher the required temperature.

The feed stream to the dehydrocyclodimerization process is defined herein as all streams introduced into the dehydrocyclodimerization reaction zone. Included in the feed stream is the C₂ -C₆ aliphatic hydrocarbon. By C₂ -C₆ aliphatic hydrocarbons is meant one or more open, straight or branched chain isomers having from two to six carbon atoms per molecule. Furthermore, the hydrocarbons in the feedstock may be saturated or unsaturated. Preferably, the hydrocarbons C₃ and/or C₄ are selected from isobutane, normal butane, isobutene, normal butene, propane and propylene. Diluents may also be included in the feed stream. Examples of such diluents include nitrogen, helium, argon, neon.

According to the present invention, the feed stream is contacted with the instant catalyst in a dehydrocyclodimerization reaction zone maintained at dehydrocyclodimerization conditions. This contacting may be accomplished by using the catalyst in a fixed bed system, a moving bed system, a fluidized bed system, or in a batch type operation; however, in view of the danger of attrition losses of the valuable catalyst and of the well-known operational advantages, it is preferred to use either a fixed bed system or a dense-phase moving bed system such as shown in U.S. Pat. No. 3,725,249.

In a fixed bed system or a dense-phase moving bed the feed stream is preheated by any suitable heating means to the desired reaction temperature and then passed into a dehydrocyclodimerization zone containing a bed of the instant catalyst. It is, of course, understood that the dehydrocyclodimerization zone may be one or more separate reactors with suitable means therebetween to assure that the desired conversion temperature is maintained at the entrance to each reactor. It is also important to note that the reactants may be contacted with the catalyst bed in either upward, downward, or radial flow fashion with the latter being preferred. In addition, the reactants are in the vapor phase when they contact the catalyst. The dehydrocyclodimerization system then preferably comprises a dehydrocyclodimerization zone containing one or more fixed or dense-phase moving beds of the instant catalyst. In a multiple bed system, it is, of course, within the scope of the present invention to use the present catalyst in less than all of the beds with another dehydrocyclodimerization or similarly behaving catalyst being used in the remainder of the beds. This dehydrocyclodimerization zone may be one or more separate reactors with suitable heating means therebetween to compensate for any endothermicity encountered in each catalyst bed. Specific to the dense-phase moving bed system, it is common practice to remove catalyst from the bottom of the reaction zone, regenerate it by conventional means known to the art, and then return it to the top of the reaction zone.

After some time on stream (several months to a year), the catalyst described above will have lost enough activity due to hydrogen exposure so that it must be reactivated. The exact amount of time which a catalyst can operate without necessitating reactivation will depend on a number of factors including whether during preparation the catalyst has been treated with an aqueous solution of a weakly acid ammonium salt or a dilute acid.

When the catalyst requires reactivation, it is removed from the operating reactor and contacted with an aqueous solution of a weakly acid ammonium salt or a dilute acid. The weakly acid ammonium salts which can be used include ammonium chloride, ammonium acetate, ammonium nitrate and mixtures thereof. The treating solution should contain from about 0.1 to about 5 moles/liter of the salt or salt mixture. Dilute acid solutions where the acid is hydrochloric, acetic, nitric or sulfuric acid can also be used. Although concentrated acids could be used, they would degrade the zeolite and the integrity of the particles as well as removing any undesirable materials. Thus, it is desirable to use dilute acids which have a molarity from about 0.1 to about 5 moles/liter. Of these treatment solutions, it is preferred to use an ammonium nitrate solution.

As described above, the treating solution is contacted with the catalyst at a temperature of about 50° to about 100° C. for a time of about 1 to about 48 hours. The catalyst is now dried and then oxidized by heating in air (calcining) at a temperature of about 500° to about 700° C. for a time of about 1 to about 15 hours. The reactivated catalyst can now be used in the dehydrocyclodimerization process described above. The catalyst can be reactivated a number of times using this procedure.

The following examples are presented in illustration of this invention and are not intended as undue limitations on the generally broad scope of the invention as set out in the appended claims.

EXAMPLE 1

This example describes the preparation of a dehydrocyclodimerization catalyst. A first solution was prepared by adding phosphoric acid to an aqueous solution of hexamethylenetetraamine (HMT) in an amount to yield a phosphorus content of the finished catalyst equal to about 11 weight percent. A second solution was prepared by adding a ZSM-5 type zeolite to enough alumina sol, prepared by digesting metallic aluminum in hydrochloric acid, to yield a zeolite content in the finished catalyst equal to about 67 weight percent. These two solutions were commingled to achieve a homogeneous admixture of HMT, phosphorus, alumina sol, and zeolite. This admixture was dispersed as droplets into an oil bath maintained at about 93° C. The droplets remained in the oil bath until they set and formed hydrogel spheres. These spheres were removed from the oil bath, water washed, air dried, and calcined at a temperature of about 482° C. A solution of gallium nitrate was utilized to impregnate the spheres to achieve a gallium content on the finished catalyst equal to about 1 weight percent. After impregnation, the spheres were dried, then heated in pure hydrogen at 580° C. for 6 hours. The spheres were finally calcined, in the presence of steam, at a temperature of about 649° C. This catalyst was identified as catalyst A.

EXAMPLE 2

Catalyst A was accelerated aged by treating the catalyst with a hydrogen/methane gas feed at 1 atmosphere and 565° C. for a period of 100 hours. The function of the methane is to act as a diluent. This hydrogen treated catalyst was identified as catalyst B.

Catalyst B was divided into two portions, one portion was tested as described below while the other portion was treated as follows before testing. About 100 cc of Catalyst B were added to a round bottom flask equipped with a condenser and containing 500 mL of a 2M ammonium nitrate aqueous solution. This mixture was heated to reflux by means of an oil bath and refluxed for 24 hours. Upon cooling the mixture was filtered and the spheres washed five times with 100 mL of deionized water. The washed spheres were dried at 150° C. for 3 hours and then calcined at 540° C. for 2 hours. This catalyst was identified as catalyst C.

The wash water from the ammonium nitrate treatment was analyzed to determine what elements were present. The analyses of the wash water are presented in Table A.

                  TABLE A                                                          ______________________________________                                                             Wash Water                                                 Element             (wt. %)                                                    ______________________________________                                         Si                  0.06                                                       Al                  0.18                                                       P                   0.09                                                       Ga                  0.00002                                                    ______________________________________                                    

As Table A shows, the species which are removed by the ammonium nitrate wash are composed primarily of aluminum and phosphorus.

EXAMPLE 3

The following test procedure was used to evaluate the dehydrocyclodimerization activity of catalysts A, B and C. A feedstock of propane was flowed through a reactor containing the catalyst to be tested. The propane was flowed through the reactor at a liquid hourly space velocity of 0.8 hr⁻¹ under a pressure of 1 atmosphere and at a reactor inlet temperature of 540° C. The conversion of propane to aromatics was calculated at various times during the testing.

The test results for catalysts A, B and C are presented in Table B.

                  TABLE B                                                          ______________________________________                                         Effect of Ammonium Nitrate Treatment on Catalyst Activity                      Time     Conversion (%)                                                        (Hours)  Catalyst A   Catalyst B                                                                               Catalyst C                                     ______________________________________                                         11       82           73        82                                             23       77           69        77                                             35       74           65        74                                             47       68           61        68                                             59       62           54        62                                             ______________________________________                                    

The results presented in Table B show that catalyst B which has been accelerated aged has lost considerable activity versus a fresh or unaged catalyst (catalyst A). However, when the deactivated catalyst is treated with ammonium nitrate (catalyst C), it has the same activity as the fresh catalyst. 

We claim as our invention:
 1. A process for reactivating a dehydrocyclodimerization catalyst comprising a zeolite having a Si:Al ratio greater than about 10 and a pore diameter of about 5-6 Å, an aluminum phosphate binder and gallium, said catalyst having been deactivated by exposure to hydrogen at a temperature above 500° C., the process comprising treating the catalyst with an aqueous solution of a weakly acidic ammonium salt or a dilute acid solution at a temperature of about 50° to about 100° C. for a time of about 1 to about 48 hours and then calcining the treated catalyst at a temperature of about 500° to about 700° C. for a time of about 1 to about 15 hours, thereby providing a reactivated catalyst.
 2. The process of claim 1 where the weakly acidic ammonium salt is selected from the group consisting of ammonium chloride, ammonium acetate, ammonium nitrate and mixtures thereof and is present in the solution in a concentration from about 0.1 to about 5 moles/liter.
 3. The process of claim 1 where the acid is selected from the group consisting of hydrochloric, acetic, nitric and sulfuric acid and is present in the solution in a concentration from about 0.1 to about 5 moles/liter.
 4. The process of claim 1 where the zeolite is a ZSM family zeolite.
 5. The process of claim 1 where the zeolite concentration ranges from about 30 to about 90 weight percent of the catalyst.
 6. The process of claim 4 where the gallium content on the catalyst varies from about 0.1 to about 5 weight percent, as the metal, of the catalyst. 